The Swedes have medalled in the pool at every world championships for nearly four decades, but are without 25-time world medallist and six-time Olympic medal winner Sarah Sjostrom, who is pregnant.
But fears of that long medal run ending came close to being put to bed on the first day in Singapore, only for Johansson to miss out on the bronze medal by two seconds.
The 26-year-old has never won a major medal in senior form, but produced the result of his career by swimming a time of 3:45.26 in the heats, a new national record.
However, that was only the sixth fastest time of the eight qualifiers, so the Nassjo man would likely need to better his performance in the final.
That is exactly what he delivered, shaving over half a second off his time to improve the Swedish record to 3:44.68.
But that was still 2.08 seconds behind South Korea's Kim Woo-min, who finished third behind the winner from Germany, Lukas Martens, whose 3:42.35 was two hundreths of a second quicker than Australia's Samuel Short, who had to settle for silver.
Johansson will be back in the pool in the coming days for the 800m and 1500m freestyles.
Elsewhere, there was disappointment for Louise Hansson, who failed to make it through the heats of the one of her strongest events, the women's 100m butterfly.
A time of 58.64 was more than 2.4 seconds down on her personal best and behind compatriot Sara Junevik, who missed the semi-finals by 0.15 seconds.
Daniel Kertes and Thilda Hall were eliminated in the heats of the men's 100m breastroke and women's 400m freestyle, respectively.